Expansion of United States and worldwide HDPE capacity is continuing unabated. Its major application is found in blow molding.
Blow molding is the most common process for making hollow parts, e.g., bottles. It is often done on a large scale with a single production line making only one product.
In blow molding processes, a molten tube of resin is extruded into a parison, an end of which is sealed die so that air can enter from one area (or end); The parison is inflated to assume the shape of the mold; and then the shaped object is cooled and ejected. Air under pressure is fed through the die into the tube which expands to fill the mold, and the part is cooled as it is held under internal air pressure. As the parison is extruded, the melt is free to swell and sag. Optimum conditions in blow molding processes require that the resin be characterized by very consistent swell and sag melt properties. A viscous melt is needed to prevent sag. Swell reduction has been achieved by controlled degradation of HDPE resins. However, controlled degradation produces thermally unstable resins which have low ESCR properties. If a very large container is to be made, the machine may be equipped with a cylinder and a piston as an accumulator for the melt. The accumulator is filled with melt from the extruder and then is emptied at a much faster rate to form a large parison; thus, sag of the molten tube is minimized Cf. KIRK OTHMER, "Plastics Processing" Vol. 18, pp. 198-199 (Third Edition.)
Resins used in plastic bottles, e.g. for HIC (household and industrial container) applications, also must exhibit environmental stress cracking resistance.
Environmental stress cracking resistance (ESCR) is measured by a standard Bell Bent Strip ESCR test method ASTM D1693. Stressed cracking resistance (F.sub.50) is the time taken for 50% of the specimens to fail (h). Herein below, testing was undertaken on 70 mils ASTM plaques with slit in 100% Igepal at 50.degree. C.